1. Field of the Invention
The invention disclosed broadly relates to the field of semiconductor fabrication, and more particularly relates to an automated process for changing parameters governing the etching and deposition processes for such fabrication.
2. Description of the Related Art
As semiconductor devices have become more highly integrated, there has been an increasing need for methods of anisotropically etching structures in the semiconductors, particularly in silicon. It is known in the art to use a reactive gas mixture containing a chemically reactive species and electrically charged particles, and to accelerate this gas mixture toward a substrate by means of an electrical prestress applied to the substrate. The charged ions, ideally, impact vertically on the substrate surface and promote a chemical reaction between the reactive gas and the substrate surface or etching base.
Favored techniques often involved using reactive gases based on fluorochemicals. This allowed a high etching rate and a high selectivity. Selectivity is the ratio of silicon etching to the mask etching. Typical masks include a photoresist layer or a SiO2 layer. The fluorine-based reactive gas, however, also resulted in undesired underetching of the mask edges. Other techniques included the use of a polymer former in the gas mixture or plasma in order to coat the side walls and thereby reduce the underetching. These techniques, however, had lower selectivities and took considerably longer because the etching rate was markedly reduced. Further, use of polymer formers often resulted in underetching at greater depths in the substrate. Other techniques utilized reactive gases based on other halogens, particularly chlorine and bromine. These processes essentially etched only on the bottom of the structure or substrate and thereby reduced underetching of the mask edges, but were extremely sensitive to moisture and required costly transfer devices.
U.S. Pat. No. 5,501,893 by Laermer et al., assigned to Robert Bosch GmbH (the xe2x80x9cBosch patentxe2x80x9d) describes these processes and their limitations in more detail, and presents an advance that overcomes some of their disadvantages. The Bosch patent teaches performing anisotropic etching, that is, vertical wall etching, in two alternating stages of etching and polymerization. The polymerization effectively coats the side walls of the structure so that the subsequent etching stage can produce deep structures having near vertical edges. The separation of the polymer and the reactive gas mixture obviates the need to consider the ratio of fluorine radicals to polymer formers, and thus the process can be optimized with respect to etching rate and selectivity without adversely affecting the anisotropy of the total process.
The Bosch patent further teaches adding low ionic energy during the polymerization stages in order to prevent the formation of the polymer on the etching base. A further aspect of the process taught in the Bosch patent (the xe2x80x9cBosch processxe2x80x9d) is a useful mask selectivity due to the fact that only low ionic energies are required.
In applying the Bosch process, the deposition and etch cycles are performed, alternatingly, with a particular set of parameters or conditions for a specified number of iterations or loops, wherein one iteration includes both an etch cycle and a deposition cycle. After the specified number of iterations, the conditions are changed and the deposition and etch cycles are performed for another specified number of iterations. This process is repeated until the desired trench is completed. The loop sets and their process conditions are referred to as xe2x80x9cprocess recipes.xe2x80x9d Typical process parameters that might be changed between loops sets include, without limitation, gas flow rate, pressure, step duration, and radio frequency (xe2x80x9cRFxe2x80x9d) power.
One of the important system capabilities incorporated specifically for adaptation of the Bosch process is looping software. This enables the system to alternate efficiently between deposition and etch cycles as called for in the Bosch patent.
The Bosch etch process thus enables etching of deep anisotropic trenches over a wide aspect ratio range. The aspect ratio can be defined as the depth of the trench divided by the width of the trench. The basic capability of this process potentially provides MicroElectroMechanical Systems (xe2x80x9cMEMSxe2x80x9d) design engineers the opportunity to apply MEMS technology in increasingly diverse areas. Many of these new applications require very precise control over trench wall profiles, including undercutting at the mask/silicon interface, as well as general profile control and variation over the entire depth of the trench.
The Bosch etch mechanism leads to certain undesirable effects given fixed aspect ratio, mask, and trench profile requirements. Among these effects is the presence of transition points on the trench walls where one set of conditions is ended and another set is initiated. Referring to FIG. 1, there is shown a trench wall 30 with two transition points 32, 34. Each of these transition points is the result of changing the process parameters.
The granularity of the transitions can be improved by increasing the number of loop sets and making smaller changes for each set. However, the number of sets of loops becomes a limiting factor because programming these process recipes is labor intensive. Additionally, even with many loop sets and small transitions between them, interruptions in the wall profile can still be detected when the trenches are viewed under high magnification. The existence and/or size of the transition points can be problematic for certain applications.
Further, as is well known in the art, trench profiles will change as a loop set, with fixed parameters, is run. The profiles tend to develop in a particular direction depending on the etch process conditions and the lengths of time that those conditions are run. A further limitation of the Bosch etch mechanism is a need to increase the number of loop sets, and to perform the labor intensive process of programming each loop set, in order to counteract these trench wall dynamics.
Both the transition points and the trench wall dynamics contribute to the creation of trench walls, or beams which do not go straight down from a mask edge. For many applications, the xe2x80x9cidealxe2x80x9d beams are parallel to each other and have a constant spacing between them regardless of the depth. Such a geometry is desirable because the capacitive performance of many devices is extremely sensitive to the spacing of the walls. The spacing needs to be consistent to provide good performance and this need becomes more critical as the density of a device is increased and the beams are brought closer together. In addition to the sensitivity of the capacitive performance, the existence of transition points and the trench wall dynamics limits the ability to control the beams and to produce a specified profile, and this limitation also becomes more severe as the density of a device is increased.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set out above.
Briefly, in accordance with one aspect of the present invention, there is provided a method for controlling a variable parameter during a processing of a semiconductor device, the method including selecting a beginning value and an ending value for the variable parameter, wherein the beginning value is not equal to the ending value; selecting a function governing how the variable parameter is to be transitioned from the beginning value to the ending value; initializing the variable parameter to the beginning value; and automatically transitioning the variable parameter according to the selected function.
Briefly, in accordance with another aspect of the present invention, there is provided a method for controlling a variable parameter during a processing of a semiconductor device, the method including selecting a criterion that describes a desired result of the processing; determining a beginning value for the variable parameter, based in part on the selected criterion; receiving during the processing of the semiconductor device an input that can be used to determine whether the variable parameter needs to be modified in order to satisfy the selected criterion; determining from the received input whether the variable parameter needs to be modified in order to satisfy the selected criterion; and modifying the variable parameter in an automated fashion in an attempt to satisfy the selected criterion.
Briefly, in accordance with other aspects of the present invention, there are provided computer program products including computer readable program code for performing each of the steps in the above methods.
Briefly, in accordance with yet other aspects of the present invention, there are provided systems for implementing the above methods.